Re: ALSA core info race condition

[Takashi Iwai <tiwai@suse.de>]

On Tue, 27 Jun 2017 21:12:18 +0200,
b_lkasam@codeaurora.org wrote:
> 
> hi ALSA team,
> there is a race condition in below API when accessing list API.
> 
> In file sound/core/info.c:
> 
> Added below patch to avoid list access of same parent node
> by two threads at same time causing list_debug crash.
> 
> diff --git a/sound/core/info.c b/sound/core/info.c
> index b5158b5..c1fd671 100644
> --- a/sound/core/info.c
> +++ b/sound/core/info.c
> @@ -747,8 +747,11 @@ snd_info_create_entry(const char *name, struct
> snd_info_entry *parent)
>  	INIT_LIST_HEAD(&entry->children);
>  	INIT_LIST_HEAD(&entry->list);
>  	entry->parent = parent;
> -	if (parent)
> +	if (parent) {
> +		mutex_lock(&parent->access);
>  		list_add_tail(&entry->list, &parent->children);
> +		mutex_unlock(&parent->access);
> +	}
>  	return entry;
>  }
> 
> Please check above logic looks fine, and help comment accordingly.

Have you ever got the actual crash?
The function is supposed to be called only at probing, and its link
base is the card object, so it's never called concurrently or the
concurrency should be managed in the caller side.

Your "fix" looks OK, but it's likely superfluous from the actual
usage.  Still it might be safer to add a protection, though.


thanks,

Takashi